Thymic function can be accurately monitored by the level of recent T cell emigrants in the circulation.

Expression of the avian chT1 thymocyte antigen persists on a subpopulation of peripheral T cells enriched in the DNA deletion circles created by alphabeta and gammadelta TCR gene rearrangements. The chT1+ cells are evenly distributed among all of the peripheral T lymphocyte compartments. The levels of chT1+ T cells in the periphery gradually decline in parallel with age-related thymic involution, and these cells disappear following early thymectomy. Experiments in which variable numbers of the 14 thymic lobes are removed in young chicks indicate a direct correlation between the levels of circulating chT1+ cells and residual thymic mass. Measurement of recent thymic emigrants in the periphery thus provides an accurate indication of thymic function.